For multistage systems having one wire common to all phases, which conductor must be grounded?

In multistage systems where one wire serves as a common connection to all phases, grounding the neutral conductor is essential for several reasons.

Primarily, the neutral conductor is the return path for the unbalanced loads in a multi-phase electrical system. If a fault occurs, such as a short circuit or equipment malfunction, grounding the neutral helps to ensure safety by providing a low-resistance path for fault currents to return to the ground. This allows protective devices (like circuit breakers) to operate more effectively, cutting off any dangerous current and mitigating the risk of electric shock or equipment damage.

Moreover, grounding the neutral stabilizes the voltage during unbalanced conditions, preventing excessive voltages on phase conductors and thus enhancing system reliability. It also aids in controlling transient overvoltages and helps to limit voltage fluctuations which could adversely affect sensitive electronic equipment.

Other options inadequately address safety and operational needs in a multistage system configuration. For instance, grounding all phase conductors could create an unsafe condition without the necessary arrangements made for fault protection, and grounding only the ground conductor does not provide the same level of risk mitigation. Grounding a non-neutral conductor does not help with balancing or protecting the system from faults in the same way as grounding the neutral conductor does